CN109478553B

CN109478553B - Image sensor with large dynamic range

Info

Publication number: CN109478553B
Application number: CN201680086484.3A
Authority: CN
Inventors: 曹培炎; 刘雨润
Original assignee: Shenzhen Genorivision Technology Co Ltd
Current assignee: Shenzhen Genorivision Technology Co Ltd
Priority date: 2016-06-21
Filing date: 2016-06-21
Publication date: 2023-10-20
Anticipated expiration: 2036-06-21
Also published as: EP3472866A1; WO2017219223A1; CN109478553A; US20220013571A1; EP3472866A4; TWI763678B; US11158663B2; TW201801519A; US20180374889A1

Abstract

Disclosed herein are systems comprising: avalanche Photodiodes (APDs) (142,242,511,611); a bias voltage source (140, 240) configured to supply a reverse bias voltage to the APD (142,242,511,611); a current meter (143,243) configured to measure a current through the APD (142,242,511,611); a controller (145, 245) configured to reduce the reverse bias voltage from a value above the breakdown voltage to a value below the breakdown voltage of the APD (142,242,511,611) when the intensity of light incident on the APD (142,242,511,611) is above the threshold, and configured to determine the light intensity above the threshold based on the current through the APD (142,242,511,611) when the reverse bias voltage is below the breakdown voltage.

Description

Image sensor with large dynamic range

[ field of technology ]

The disclosure herein relates to image sensors, and more particularly, to image sensors having a large dynamic range.

[ background Art ]

An image sensor or imaging sensor is a sensor that can detect the spatial intensity distribution of radiation. The image sensor typically represents the detected image by an electrical signal. Image sensors based on semiconductor devices can be divided into several types: semiconductor Charge Coupled Devices (CCDs), complementary Metal Oxide Semiconductors (CMOS), N-type metal oxide semiconductors (NMOS). CMOS image sensors are a type of active pixel sensor fabricated using CMOS semiconductor processes. Light incident on a pixel in a CMOS image sensor is converted into a voltage. The voltage is digitized into discrete values representing the intensity of light incident on the pixel. An Active Pixel Sensor (APS) is an image sensor that includes a pixel having a photodetector and an active amplifier. The CCD image sensor includes a capacitor in the pixel. When light is incident on a pixel, the light generates charges and these charges are stored in a capacitor. The stored charge is converted to a voltage and the voltage is digitized into discrete values representing the intensity of light incident on the pixel.

The dynamic range of an image sensor is the range between the minimum and maximum light intensities that the image sensor can detect. That is, the image sensor cannot distinguish between different light intensities outside the dynamic range.

[ invention ]

Disclosed herein are systems comprising: avalanche Photodiodes (APDs); a bias voltage source configured to supply a reverse bias voltage to the APD; a current meter configured to measure a current through the APD; a controller configured to reduce the reverse bias voltage from a value above the breakdown voltage to a value below the breakdown voltage of the APD when the intensity of light incident on the APD is above the threshold, and configured to determine the light intensity above the threshold based on a current through the APD when the reverse bias voltage is below the breakdown voltage.

According to an embodiment, the controller is configured to quench the APD after the controller detects a rising edge in the current when the reverse bias voltage is above the breakdown voltage.

According to an embodiment, the controller is configured to increase the reverse bias voltage above the breakdown voltage after quenching the APD.

According to an embodiment, the controller is configured to determine an intensity of light incident on the APD based on a number of pulses in the current for a specified amount of time when the reverse bias voltage is above the breakdown voltage.

Disclosed herein is an image sensor, including: an APD array; an electronic system configured to independently control a reverse bias voltage on the APD based on an intensity of light incident on the APD.

According to an embodiment, the electronic system is configured to set reverse bias voltages differently for different APDs in the array.

According to an embodiment, the APDs are configured such that a first one of the APDs operates in a linear mode and a second one of the APDs operates in a Geiger mode at a specified time.

According to an embodiment, an electronic system is configured to determine an intensity of light incident on an APD operating in a linear mode and an intensity of light incident on an APD operating in a Geiger mode.

According to an embodiment, an electronic system is configured to cause APDs in an array that are exposed to light intensities above their saturation intensities to operate in a linear mode; wherein the electronic system is configured to cause the APDs in the array to be exposed to light intensities below their saturation intensities to operate in Geiger mode.

According to an embodiment, the electronic system is configured to switch APDs in the array independently between operating in a linear mode and operating in a Geiger mode based on the intensity of light incident on the APDs.

According to an embodiment, the image sensor is configured to output a representation of the intensity of light incident on the APD without communicating an operational mode of the APD to downstream circuitry.

According to an embodiment, the APD is in or on a first substrate and the electronic system is in or on a second substrate; wherein the first substrate and the second substrate are bonded together.

According to an embodiment, the image sensor further comprises a transmission line in the first substrate or in the second substrate.

According to an embodiment, the image sensor further comprises a via configured to electrically connect the APD and the electronic system.

Disclosed herein is a telescopic scope including an image sensor disposed therein.

Disclosed herein are night vision goggles that include an image sensor disposed therein.

Disclosed herein is a telescope including an image sensor disposed therein.

Disclosed herein are spectrometers that include an image sensor disposed therein.

Disclosed herein are vehicles, including an image sensor disposed therein, wherein the vehicle is a land vehicle, a spacecraft, an aircraft, or a water surface vehicle.

Disclosed herein are methods of using APDs, comprising: (a) Applying a first reverse bias voltage to the APD that is above the breakdown voltage of the APD; (b) measuring a first intensity of light incident on the APD; (c) determining whether the first intensity is above a first threshold; repeating (a) - (c) if the first intensity is not above the first threshold; if the first intensity is above the first threshold: (d) Applying a second reverse bias voltage to the APD that is below the breakdown voltage; (e) measuring a second intensity of light incident on the APD; (f) determining whether the first intensity is below a second threshold; repeating (d) - (f) if the second intensity is not below the second threshold; if the second intensity is below the first threshold, performing (a) - (c).

According to an embodiment, measuring the first intensity includes counting a number of current pulses through the APD in a given amount of time.

According to an embodiment, measuring the second intensity includes measuring a current in the APD.

According to an embodiment, the first threshold is a saturation intensity of the APD.

According to an embodiment, the first and second threshold values are the same.

[ description of the drawings ]

Fig. 1A schematically shows current-voltage characteristics of APDs employing a linear mode and employing a Geiger mode.

FIG. 1B schematically shows a function of current in an APD as a function of intensity of light incident on the APD when the APD is in a linear mode and a function of current in the APD as a function of intensity of light incident on the APD when the APD is in a Geiger mode.

Fig. 1C schematically shows the current through SPAD as a function of time.

Fig. 1D schematically illustrates a circuit, which includes SPADs.

Fig. 2 illustrates a system according to an embodiment, including an APD.

Fig. 3 schematically shows a flow chart of a method of using an APD according to an embodiment.

Fig. 4 schematically illustrates a top view of an image sensor (which includes an APD array).

Fig. 5A and 5B schematically illustrate cross-sectional views of an image sensor (which includes a plurality of APDs).

Fig. 6A and 6B schematically illustrate cross-sectional views of an image sensor (which includes a plurality of APDs).

Fig. 7 schematically illustrates a night vision telescopic scope including an image sensor disposed therein.

Fig. 8 schematically illustrates a pair of night vision goggles including an image sensor disposed therein.

Fig. 9 schematically illustrates a telescope including an image sensor disposed therein.

[ detailed description ] of the invention

Single Photon Avalanche Diodes (SPADs) (also known as Geiger mode APDs or G-APDs) are Avalanche Photodiodes (APDs) that operate at reverse bias voltages above breakdown voltage. The word "above" herein means that the absolute value of the reverse bias is greater than the absolute value of the breakdown voltage. When a photon is incident on SPAD, it can generate carriers (electrons and holes). Some of the carriers are accelerated by the electric field in SPADs and can trigger avalanche current by impact ionization. Impact ionization is the process by which one energetic carrier in a material can lose energy by creating other carriers. For example, in a semiconductor, an electron (or hole) with sufficient kinetic energy may knock a bound electron out of its bound state (in the valence band) and raise it to a state in the conduction band, creating an electron-hole pair. SPADs can be used to detect low intensity light (e.g., down to a single photon) and signal the arrival time of a photon with tens of picosecond jitter.

SPADs take the form of p-n junctions at reverse bias voltages above the breakdown voltage of the p-n junction (i.e., the p-type region of the p-n junction is biased at a lower potential than the n-type region). The breakdown voltage of the p-n junction is the reverse bias voltage above which an exponential increase in current occurs in the p-n junction.

Fig. 1A schematically illustrates the current-voltage characteristics 100 of an APD in a linear mode and in a Geiger mode (i.e., when the APD is SPAD). APD can have a divergence of current-voltage characteristics 100 above breakdown voltage VBD (i.e., SPAD). At reverse bias voltages above VBD, both electrons and holes can cause significant ionization and avalanche self-sustaining. When avalanche is triggered (e.g., by an incident photon) at a reverse bias above VBD, the avalanche current continues ("open branch" 110); when avalanche is not triggered at a reverse bias above VBD, little current flows ("off-branch" 120). At reverse bias voltages above VBD, when an incident photon triggers an avalanche in the APD, the APD's current-voltage characteristic 100 transitions (as indicated by arrow 130) from the off-branch 120 to the on-branch 110. This transition is manifested as a sharp increase in current through the APD from substantially zero to a finite value IL. This transition is similar to the mechanism behind the Geiger counter. Thus, at reverse bias voltages higher than VBD, the APDs operate in "Geiger mode". APDs operating at reverse bias voltages below the breakdown voltage operate in a linear mode because the current in the APD is proportional to the intensity of the light incident on the APD.

FIG. 1B schematically shows a function 112 of current in an APD with respect to the intensity of light incident on the APD when the APD is operated in a linear mode and a function 111 of current in an APD with respect to the intensity of light incident on the APD when the APD is operated in a Geiger mode (i.e., when the APD is SPAD). In Geiger mode, the current shows a very sharp increase with light intensity and then saturates. In the linear mode, the current is substantially proportional to the intensity of the light.

Fig. 1C schematically shows the current through SPAD as a function of time. When light is incident on the SPAD and triggers an avalanche, a steep rising edge 131 of the current-time (I-t) curve occurs. The current increases rapidly from substantially zero to a finite value IL. The current remains at the finite value IL until the reverse bias on SPAD is redirected to substantially zero. Resetting the reverse bias on SPADs to substantially zero may be referred to as "quenching" SPADs. SPAD quenching was presented as a falling edge 132 in the I-t curve.

Fig. 1D schematically illustrates a circuit that includes SPAD142 (i.e., APD operating in Geiger mode). The circuit is configured to quench SPAD 142. The bias source 140 supplies a reverse bias to the SPAD142 through the switch 141. The current through SPAD142 is measured by current meter 143. SPAD142 is connected to ground 144 by ammeter 143. The current measured by the ammeter 143 is transmitted to the controller 145. The controller 145 is configured to quench the SPAD 142. In an example, after the controller 145 detects a rising edge (e.g., rising edge 131) in the current measured by the current meter 143, the controller 145 quenches the SPAD142 by opening the switch 141 thereby disconnecting the bias source 140 from the SPAD; after quenching SPAD142, controller 145 closes switch 141, after which SPAD is ready to detect the next incident photon. The dynamic range of the device shown in fig. 1D is relatively small. SPAD142 is saturated when the average time interval between two consecutive photons incident on SPAD142 is the same as or shorter than the sum of the time 146 (see fig. 1C) taken by controller 145 to quench SPAD142 (e.g., by opening switch 141) after sensing rising edge 131 and the time taken by controller 145 to recover the reverse bias (e.g., by closing switch 141) after controller 145 has quenched SPAD 142. That is, when SPAD142 is saturated, SPAD142 cannot distinguish between different intensities of incident light. When SPAD142 is not saturated, the intensity of the incident light may be obtained from the number of pulses, the number of rising edges, or the number of falling edges in a specified amount of time.

Fig. 2 illustrates a system according to an embodiment that includes APD 242. Bias voltage source 240 supplies a reverse bias voltage to APD 242. The current through APD242 is measured by ammeter 243. APD242 is connected to ground 244 by ammeter 243. The current measured by the ammeter 243 is transmitted to the controller 245. Controller 245 controls the reverse bias voltage applied to APD 242. When the reverse bias voltage applied to APD242 is higher than VBD, i.e., when APD242 is SPAD242, controller 245 is configured to quench SPAD 242. In an example, after controller 245 detects a rising edge in the current measured by ammeter 243, controller 145 quenches SPAD242 by disconnecting SPAD242 from bias source 240 or setting the reverse bias to substantially zero (e.g., below 0.1V); the controller 245 causes the reverse bias voltage to go back above the breakdown voltage VBD after quenching the SPAD242, after which the SPAD242 is ready to detect the next incident photon. The controller 245 is also configured to sense the intensity of light incident on the SPAD 242. When the intensity is above a threshold (e.g., when the intensity saturates SPAD 242), i.e., when SPAD242 is an APD242 operating in a linear mode, controller 245 reduces the reverse bias voltage on SPAD242 to a smaller value below VBD. Herein, the phrase "reducing the reverse bias" means reducing the absolute value of the reverse bias; the word "smaller" as used with respect to reverse bias means that the absolute value of reverse bias is smaller. The controller 245 is configured to sense the intensity of light incident on the APD242 in a linear mode. When the intensity is below the threshold (i.e., when the intensity does not result in saturation (if the reverse bias voltage increases above VBD), the controller 245 increases the reverse bias voltage on APD242 to a greater value above VBD (i.e., APD242 is now SPAD 242).

Fig. 3 schematically shows a flow chart of a method of using an APD according to an embodiment. In process 310, a reverse bias voltage V1 is applied to the APD that is above the breakdown voltage VBD of the APD. In process 320, the intensity of light incident on the APD is measured. For example, when the APD is a SPAD at V1, the intensity can be measured by counting the number of current pulses through the APD in a specified amount of time. In process 330, it is determined whether the intensity measured in process 320 is above a first threshold. For example, the first threshold may be an intensity that results in SPAD saturation. If the intensity is not above the first threshold, flow returns to process 310. If the intensity is above the first threshold, flow proceeds to process 340. In process 340, a reverse bias voltage V2 is applied to the APD that is below the breakdown voltage VBD. In process 350, the intensity of light incident on the APD is measured. For example, when the APD is not SPAD at V2, the intensity can be measured by measuring the current through the APD. In process 360, it is determined whether the intensity measured in process 350 is below a second threshold. For example, the first threshold may be an intensity that does not cause SPAD saturation at the reverse bias V1. If the intensity is not below the second threshold, flow returns to process 340. If the intensity is below the second threshold, flow proceeds to process 310. The first and second thresholds may be the same or different.

Fig. 4 schematically illustrates a top view of an image sensor 400 (which includes an array 410 of APDs). Image sensor 400 has an electronic system (which includes, for example, one or more of controllers 245) configured to independently control the reverse bias voltage on the APD based on the intensity of light incident on the APD. The electronics can be configured to set the reverse bias voltages differently for different APDs in the array 410. At a given time, some of the APDs in array 140 may operate in a linear mode, and some may operate in a Geiger mode (i.e., SPAD). The electronic system can be configured to determine the intensity of light incident on the APD, regardless of whether the APD is operating in a linear mode or a Geiger mode. Image sensor 400 thus has a combined dynamic range of APDs operating in a linear mode and APDs operating in a Geige mode. When image sensor 400 is exposed to a scene having a portion of high light intensity that will saturate an APD operating in Geiger mode, those APDs in the array that are exposed to that portion can operate in linear mode and the remaining APDs can operate in Geiger mode. The incident light intensity above which an APD operating in Geiger mode is saturated is referred to as the "saturation intensity" of the APD. APDs in the array can be controlled using the method illustrated in fig. 3. The electronic system can switch APDs in the array independently between operating in a linear mode and in a Geiger mode based on the intensity of light incident on the APDs as the scene changes. Image sensor 400 may be configured to output a representation of the intensity of light incident on the APD without having to pass the operational mode of the APD to downstream circuitry. The image sensor 400 may be configured to sense a scene of infrared light, visible light, ultraviolet light, or X-rays.

Fig. 5A and 5B schematically illustrate cross-sectional views of an image sensor 500 (which includes a plurality of APDs 511). APD 511 may be fabricated in a substrate 510 (e.g., a semiconductor wafer). One or more vias 512 may be present in the substrate 510 and the vias 512 electrically connect the APDs 511 to the surface of the substrate 510. Alternatively, APD 511 may be disposed on a surface of substrate 510 such that electrical contacts on APD 511 are exposed to the surface. Electronic system 521, which communicates with APD 511 and/or controls APD 511, may be fabricated in another substrate 520. Electronic system 521 may include a controller, bias source, switch, ammeter, memory, amplifier, or other suitable component. Some components of the electronic system 521 may be fabricated in the substrate 510. Electronic system 521 can be configured to use APD 511 using the method illustrated in fig. 3. One or more vias 522 may be present and electrically connect the electronic system 521 to the surface of the substrate 520. Alternatively, the electronic system 521 may be disposed at the surface of the substrate 520 such that electrical contacts on the electronic system 521 are exposed at the surface. Substrate 520 can include transmission line 530 configured to transmit data, power, and/or transport to and from electronic system 521, and through to APD 511 and from APD 511. Substrates 510 and 520 may be bonded by suitable substrate bonding, such as flip-chip bonding or direct bonding.

As shown in fig. 5A and 5B, flip-chip bonding uses solder bumps 599 deposited onto the surface of either of substrates 510 and 520. Either of the substrates 510 and 520 is flipped and APD 511 and electronic system 521 are aligned (e.g., through vias 512, 522, or both). The substrates 510 and 520 are urged into contact. Solder bumps 599 can be melted to electrically connect APD 511 and electronic system 521. Any void spaces between solder bumps 599 may be filled with an insulating material.

Direct bonding is a wafer bonding process without any additional intermediate layers (e.g., solder bumps). The bonding process is based on chemical bonding between two surfaces. Direct bonding may be performed at elevated temperatures but is not required to be so.

Fig. 6A and 6B schematically illustrate cross-sectional views of an image sensor 600 (which includes a plurality of APDs 611). APD 611 may be fabricated in a substrate 610 (e.g., a semiconductor wafer). One or more vias 612 may be present in the substrate 610 and the vias 612 electrically connect the APDs 611 to the surface of the substrate 610. Alternatively, APD 611 may be disposed on a surface of substrate 610 such that electrical contacts on APD 611 are exposed to the surface. The substrate 610 may include a transmission line 630. The electronic system 621 in communication with APD 611 and/or controlling APD 611 can be fabricated in another substrate 620. The electronic system 621 may include a controller, bias source, switch, ammeter, memory, amplifier, or other suitable component. Some components of the electronic system 621 may be fabricated in the substrate 610. The electronic system 621 can be configured to use APD 611 using the method illustrated in fig. 3. One or more vias 622 and 623 may be present and electrically connect the electronic system 621 to the surface of the substrate 620. Alternatively, the electronic system 621 may be disposed at the surface of the substrate 620 such that electrical contacts on the electronic system 621 are exposed to the surface. Substrates 610 and 620 may be bonded by suitable substrate bonding techniques such as flip-chip bonding or direct bonding.

As shown in fig. 6A and 6B, flip-chip bonding uses solder bumps 699 and 698 deposited onto the surface of either of the substrates 610 and 620. Either of the substrates 610 and 620 is flipped and APD 611 and electronic system 621 are aligned (e.g., through vias 612, 622, or both). The substrates 610 and 620 are urged into contact. Solder bumps 699 can be melted to electrically connect APD 611 and electronic system 621. The solder bumps 698 may be melted to electrically connect the electronic system 620 to the transmission line 630. Transmission line 630 is configured to transmit data, power, and/or signals to and from electronic system 621, and through to APD 611 and from APD 611. Any void space between solder bumps 599 and 698 can be filled with an insulating material.

Fig. 7 schematically illustrates a night vision telescopic scope 700 that includes an image sensor 730 (e.g., image sensor 400, 500, or 600) disposed therein. The scope 700 includes one or more optical (refractive or reflective) components 710 that project a scene to an image sensor 730. The image sensor 730 generates an electronic signal representing the scene. These electronic signals are transmitted to the display 740. The display 740 displays an image based on the electronic signal. The scope 700 may include one or more optical (refractive or reflective) components 720 configured to project an image to a person using the scope.

Fig. 8 schematically illustrates a pair of night vision goggles 800 that include an image sensor 830 (e.g., image sensor 400, 500, or 600) disposed therein. Mirror 800 includes one or more optical (refractive or reflective) components 810 that project a scene to an image sensor 830. The image sensor 830 generates an electronic signal representing the scene. These electronic signals are transmitted to one or both displays 840. Each of the displays 840 displays an image based on the electronic signals. Mirror 800 can include one or more optical (refractive or reflective) components 820 configured to project an image to a person using the mirror.

Fig. 9 schematically illustrates a telescope 900 that includes an image sensor 930 (e.g., image sensor 400, 500, or 600) disposed therein. Telescope 900 includes one or more optical (refractive or reflective) components 910 that project a scene to an image sensor 930. The image sensor 930 generates an electronic signal representing the scene. These electronic signals are transmitted to one or both displays and/or captured for analysis.

The spectrometer may include an image sensor (e.g., image sensor 400, 500, or 600) disposed therein. The spectrometer uses a prism or grating to spread light from the scene into a spectrum. The spectrum may be projected to an image sensor for detection.

A vehicle (e.g., a land vehicle, spacecraft, aircraft, water surface vehicle) may include an image sensor (e.g., image sensor 400, 500, or 600) disclosed herein.

While various aspects and embodiments are disclosed herein, other aspects and embodiments will become apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims

1. An image sensing system, comprising:

avalanche photodiodes APD;

a bias voltage source configured to supply a reverse bias voltage to the APD;

a current meter configured to measure a current through the APD;

a controller configured to reduce the reverse bias voltage from a value above a breakdown voltage to a value below the breakdown voltage of the APD when the intensity of light incident on the APD is above a threshold, thereby operating the APD in a linear mode, and configured to determine a light intensity above the threshold based on a current through the APD when the reverse bias voltage is below the breakdown voltage.

2. The system of claim 1, wherein the controller is configured to quench the APD after the controller detects a rising edge in the current when the reverse bias voltage is above the breakdown voltage.

3. The system of claim 2, wherein the controller is configured to increase the reverse bias voltage above the breakdown voltage after quenching the APD.

4. The system of claim 1, wherein the controller is configured to determine the intensity of light incident on the APD based on the number of pulses in the current in a specified amount of time when the reverse bias voltage is above the breakdown voltage.

5. An image sensor, comprising:

an avalanche photodiode APD array;

an electronic system configured to independently control reverse bias voltages on the APDs based on the intensity of light incident on the APDs and configured to independently switch APDs in the array between operating in a linear mode and operating in a Geiger mode based on the intensity of light incident on the APDs.

6. The image sensor of claim 5 wherein the electronic system is configured to set the reverse bias voltage differently for different APDs in the array.

7. The image sensor of claim 5, the APD comprising a first APD and a second APD, wherein the APD is configured such that at a specified time the first APD operates in a linear mode and the second APD operates in a Geiger mode.

8. The image sensor of claim 5 wherein the electronic system is configured to determine an intensity of light incident on the APD operating in a linear mode and an intensity of light incident on the APD operating in a Geiger mode.

9. The image sensor of claim 5, wherein the electronic system is configured to cause APDs in the array that are exposed to light intensities above a saturation intensity of the APDs to operate in a linear mode; wherein the electronic system is configured to cause APDs in the array that are exposed to light intensities below a saturation intensity of the APDs to operate in Geiger mode.

10. The image sensor of claim 5, wherein the image sensor is configured to output a representation of an intensity of light incident on the APD without passing an operational mode of the APD to downstream circuitry.

11. The image sensor of claim 5 wherein the APD is in or on a first substrate and the electronic system is in or on a second substrate; wherein the first substrate and the second substrate are bonded together.

12. The image sensor of claim 11, further comprising a transmission line in the first substrate or in the second substrate.

13. The image sensor of claim 5, further comprising a via configured to electrically connect the APD and the electronic system.

14. A telescopic sight comprising an image sensor as claimed in claim 5.

15. A night vision goggle comprising the image sensor of claim 5.

16. A telescope comprising the image sensor of claim 5.

17. A spectrometer comprising the image sensor of claim 5.

18. A traffic vehicle comprising the image sensor of claim 5, wherein the vehicle is a land vehicle, a spacecraft, an aircraft, or a water surface vehicle.

19. A method of using an avalanche photodiode APD, comprising:

(a) Applying a first reverse bias voltage to the APD that is higher than a breakdown voltage of the APD;

(b) Measuring a first intensity of light incident on the APD;

(c) Determining whether the first intensity is above a first threshold;

repeating (a) - (c) if the first intensity is not above the first threshold;

if the first intensity is above the first threshold:

(d) Applying a second reverse bias voltage to the APD that is lower than the breakdown voltage;

(e) Measuring a second intensity of light incident on the APD;

(f) Determining whether the first intensity is below a second threshold;

repeating (d) - (f) if the second intensity is not below the second threshold;

if the second intensity is below a first threshold, performing (a) - (c).

20. The method of claim 19, wherein measuring the first intensity comprises counting a number of current pulses through the APD in a given amount of time.

21. The method of claim 19, wherein measuring the second intensity comprises measuring a current in the APD.

22. The method of claim 19, wherein the first threshold is a saturation intensity of the APD.

23. The method of claim 19, wherein the first and second thresholds are the same.